Because of their excellent color, hardness, and solvent, water, and chemical resistance, melamine resins are often used in the saturation of decorative and overlay paper for the preparation of decorative laminates. They are also resistant to heat and abrasion. There are many patents describing the use of melamine resins in these applications. Unfortunately, unmodified melamine resins have very short shelf lives, e.g., no more than about 1-3 days, after which time resin begins to separate from solution. Consequently, such resins must be used shortly after their preparation, and unexpected scheduling disruption can lead to loss of product. Laminates produced from such unmodified melamine resins also tend to be very brittle and normally cannot be post-formed to produce clear, crack-free surfaces.
The art has recognized that many of the shortcomings of unmodified melamine resins can be addressed by the introduction of various resin modifiers. Modifiers are available for increasing the shelf life of melamine resins, for increasing flow during processing, for increasing the cured resin's flexibility, and/or for increasing post-formability of a laminated product. Often, a plasticizer is added to melamine formulations to improve shelf life and to aid post-formability. Plasticizers which have been used in this regard include sucrose, sorbitol, alkylguanamines, aliphatic polyols, aromatic amides, aromatic amines, polyvinyl alcohol, monocarbamates, caprolactam, and isocyanuarates. U.S. Pat. No. 3,716,395, for example, describes a laminated material which contains, inter alia, a coating layer comprised of, for example, melamine- and/or urea-formaldehyde resins, a pigment, and a plasticizer, such as a mixture of sugar and sorbitol.
There are many patents directed to modified melamine resins. For instance, U.S. Pat. No. 3,832,316 discloses aqueous resin solutions formed by partially reacting formaldehyde, melamine, and dicyandiamide in the presence of an oxy-acid of phosphorous. U.S. Pat. No. 3,194,720 describes laminating resins for producing rigid and decorative post-formable laminates. The resins comprise the co-condensation product of an aminotriazine, such as melamine, and an aldehyde. An anhydride of sorbitol is also included to enhance the stability of the aqueous resin solution. U.S. Pat. No. 5,346,937 relates to melamine resins modified by dicyandiamide, a water soluble starch, and a guanamine derivative which resins are useful for decorative paper laminates.
WO 96/20230 discloses resins for use in post-formed laminates prepared by reacting formaldehyde, melamine, dicyandiamide, and a polyol such as trimethylolpropane, 1,4-cyclohexanedimethanol, or 1,6-hexanediol. BE 1005822 A3 discloses melamine-formaldehyde resins modified with dicyandiamide. The resins are useful for impregnation of paper for making laminates.
Unfortunately, the steps taken by the prior art to improve post-formability often create laminates whose color, clarity, or chemical and water resistance are adversely affected. For instance, sugar-modified melamine-formaldehyde resins often yellow when subjected to heat and pressure as the laminate is cured. Other modifiers in the prior art also reduce the boiling water resistance of the cured laminate while increasing the flexibility of the laminate. Still other modifiers reduce the shelf life of the resin or reduce the flexibility and gloss characteristics of the final laminate to unacceptable levels.
It was thus desired to obtain a modifier for melamine resins which does not have the disadvantages of the prior art modifiers, improves resin storage stability, and confers good flexibility and gloss to the cured laminates.